Fractality as a quantitative assessment tool for tic disorders and functional tic-like behaviors

Washington University, Saint Louis MO

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Abstract

Project summary The natural history of tics is characterized by a high prevalence in boys, with onset around around 4‒6 years of age. The initial tic symptoms are usually simple motor tics affecting the face, head, and neck. The first vocal tic occurs on average a few years later, and is usually a simple vocal tic such as throat clearing or sniffing. If the tics persist more than a year, the severity usually peaks around the age of 8 to 12 years. Most individuals with tics undergo significant improvement or complete resolution before early adulthood. Recently, while tics in patients with Tourette syndrome (TS) have been increasing rapidly, a different pattern has also been observed. Some teenagers and young adults have experienced sudden onset of complex tic-like movements and vocalizations. These patients differ significantly on several other features as well, and such distinct tic-like behavior is now referred to as functional neurological symptom disorder with tics, or functional tic-like behavior (FTLB). However, differentiating FTLB from typical tics is still challenging, and diagnosis and treatment are important issues under active study in numerous centers worldwide. Tic disorders are distinguished from other movement disorders not only by the phenomenology of individual tics, but also by the timing of tics: they are neither rhythmic nor completely unpredictable. Rather, tics tend to occur in clusters (several tic attacks within a few seconds, followed by an extended period of tic-free time). On longer time scales, several bouts of tics may be followed by a relatively tic-free period with repeated episodes grouped over minutes, and patients often describe a similar pattern over days or weeks. Based on this self- similarity over different time scales, fractal timing of tics in TS was demonstrated over 20 years ago, and we recently extended those results to both Provisional Tic Disorder and TS, applying a model from physics to measure fractal dimension (Df) in relatively brief data sets (5 minutes). The Df of the tic time series correlates with the tic severity, suggesting that Df may be useful as an objective indicator of tic severity. We hypothesized that the fractal dimension Df of tics may distinguish FTLB from typical tics. Specifically, FTLB may not show a fractal pattern, or the quantitatively measured fractal dimension may differ from that of typical tics. The objective of this proposal is to investigate Df with the following specific aim: Df will be computed from video samples of tics or FTLB, blind to diagnosis, contributed locally and from collaborators worldwide, and compared to test this hypothesis. The long-term goal is to find an objective measure that distinguishes FLTB from tic disorders, and if Df can be successfully demonstrated to differ between the two, future studies could look at comorbidities, treatment, and how they change when healthy individuals mimic tics.